The immune response comprises a cellular response and a humoral response. The cellular response is mediated largely by T lymphocytes (alternatively and equivalently referred to herein as T-cells), while the humoral response is mediated by B lymphocytes (alternatively and equivalently referred to herein as B-cells).
B-cells produce and secrete antibodies in response to the presentation of antigen and MHC class II molecules on the surface of antigen presenting cells. Antigen presentation initiates B-cell activation with the engagement of the B-cell receptor (BCR) at the cell's surface. Following engagement, the BCR relays signals that are propagated through the cell's interior via signal transduction pathways. These signals lead to changes in B-cell gene expression and physiology, which underlie B-cell activation.
T-cells produce costimulatory molecules, including cytokines, that augment antibody production by B-cells during the humoral immune response. Cytokines also play a role in modulating the activity of T-cells themselves. Many T-cells act directly to engulf and destroy cells or agents that they recognize by virtue of the cell surface proteins they possess. The engagement of cell surface receptors on T-cells results in the propagation of intracellular signals that provoke changes in T-cell gene expression and physiology, which underlie the cellular immune response.
Antigen recognition alone is usually not sufficient to initiate a complete effector T or B-cell response. The generation of many B-cell responses to antigen is dependent upon the interaction of B-cells with CD4+ helper T-cells directed against the same antigen. These helper T-cells express CD40L (CD154) which binds to the cell surface receptor, CD40, on resting B-cells. This interaction provides a critical activation signal to B-cells. Mutations in the CD40L lead to the X-linked immunodeficiency disorder hyper-IgM syndrome, which is characterized by low levels of IgA and IgG, normal to elevated levels of IgM, absence of germinal center formation, and decreased immune response. In addition, transgenic mice lacking CD40 exhibit reduced graft rejection. (Zanelli et al., Nature Medicine, 6: 629-630, 2000; Schonbeck et al., Cell Mol Life Sci, 58:443, 2001).
Intercellular communication between different types of lymphocytes, as well as between lymphocytes and non-lymphocytes in the normally functioning immune system is well known. Much of this communication is mediated by cytokines and their cognate receptors. Cytokine-induced signals begin at the cell surface with a cytokine receptor and are transmitted intracellularly via signal transduction pathways. Many types of cells produce cytokines, and cytokines can induce a variety of responses in a variety of cell types, including lymphocytes. The response to a cytokine can be context-dependent as well as cell type specific.
Dysregulation of intercellular communication can perturb lymphocyte activity and the regulation of immune responses. Such dysregulation is believed to underlie certain autoimmune disease states, hyper-immune states, and immune-compromised states. Such dysfunction may be cell autonomous or non-cell autonomous with respect to lymphocytes.
The activation of specific signaling pathways in lymphocyte determines the quality, magnitude, and duration of immune responses. In response to transplantation, in acute and chronic inflammatory diseases, and in autoimmune responses, it is these pathways that are responsible for the induction, maintenance and exacerbation of undesirable lymphocyte responses. Identification of these signaling pathways is desirable in order to provide diagnostic and prognostic tools, as well as therapeutic targets for modulating lymphocyte function in a variety of disorders or abnormal physiological states. In addition, the ability to modulate these pathways and suppress normal immune responses is often desirable, for example in the treatment of hosts receiving a transplant.
The cytoskeleton is a target of some signal transduction pathways and regulation of the actin cytoskeleton is an important point of control in the immune response. The migration of lymphocytes in response to chemokines, the division of lymphocytes in response to cytokines and antigens, and the cellular shape changes associated with the development of plasma cells from pre B-cells, all involve changes in the actin cytoskeleton.
Myosin proteins are important regulators of actin organization, as well as motor proteins which interact with actin filaments to mediate important cellular functions, e.g., vesicle trafficking.
Unconventional myosins make up a diverse group of multidomain actin-based motor proteins which have been implicated in the regulation of focal actin polymerization and the trafficking of actin and phospholipids along actin fibers. The class I myosins contain an N-terminal myosin head domain, comprising an ATP-binding motif and an actin binding site. The myosin head domain has ATPase activity and exhibits ATP-dependent actin binding activity. Following the myosin head domain is an IQ domain(s), which mediates binding to the calcium-binding protein “calmodulin”. Following the IQ domain are three domains, denoted TH1, TH2 and TH3 (Crozet-et al., Genomics; 40: 332-341, 1997).
The TH1 domain is rich in basic residues and mediates myosin-l binding to phospholipds. The TH2 domain is enriched in glycine, proline and alanine, and may mediate ATP-independent binding to actin. The C-terminal TH3 domain is an SH3 domain, which mediates protein-protein interactions (Crozet et al., supra).